Macrophage colony-stimulating factor (M-CSF or CSF-1) is a cytokine that affects the proliferation and differentiation of hematopoietic stem cells and is involved in maintaining and augmenting the survival, effector functions and cytokine secretion of human monocytes. Secretion of M-CSF by activated monocytes suggests that the stimulation of effector functions by this cytokine is regulated through autocrine or paracrine mechanisms. We investigated the conditions and stimuli that cause monocytes to produce M-CSF. Peripheral blood monocytes isolated by elutriation were cultured in suspension culture in polypropylene tubes. PMA was a potent inducer of H-CSF message measured at 15 hours. Granulocyte-macrophage colony-stimulating factor (GM-CSF) and to a lesser extent tumor necrosis factor (TNF alpha) were also capable of inducing M-CSF message. IFN gamma induced low levels of M-CSF message after 2 hours but no message was detected after 15 hours. Although several different CSF-1 mRNAs can be generated as a result of alternative splicing, only the 4 KB CSF-1 mRNA was detectable in human monocytes by any tested stimulus. Lipopolysaccharide (LPS) did not induce any M-CSF message although it is known to be a potent monocyte stimulant for the induction of other cytokines such as IL-1 and TNF alpha. IL-4 and M-CSF were also negative in inducing M-CSF message levels in monocytes. Secretion of M-CSF was assayed in monocyte supernatants by measuring the proliferation of the M-NFS60 and 32Dcfms murine cell lines. This confirmed that PMA and GM-CSF, and to a much lesser extent, TNF stimulated M-CSF production. These results will help to understand the role of monocytes and their products in the complex cytokine regulatory network.